False ceiling

ABSTRACT

A false ceiling for buildings designed to absorb acoustic waves has  perfoed plates. One or several suspended plates (1, 6) are provided which are so hard that they cannot vibrate. The plates have a plurality of regularly or irregularly arranged holes (4, 7) with 0.2-3 mm diameter, the surface of the holes being less than 4% of the total surface. The air in the holes (4, 7) forms with the overlying cavities (11) a dampening active mass system of the foil absorber type.

BACKGROUND OF THE INVENTION

The present invention relates to a false ceiling, as is known fromFrick, O., et al., "Baukonstruktionslehre", Part 1., Teubner, Stuttgart1992.

1. Subject Matter

Preferably light, for the most part prefabricated, dry and easy to mountceiling system are being widely employed in a great variety of ways assubconstructions "suspended" from massive, bearing ceilings. In newbuildings and in refurbishing lobbies of old buildings, administrativehalls, classrooms or industrial, fair or sport halls as well as officebuildings, department stores and hospitals, so-called ceiling fronts andfalse ceilings (FC) have assumed both decorative and constructionfunctions.

2. Purpose and Function

Mounted at a certain distance from the massive ceiling as panelling, theFC often helps meet various physical construction requirements in thebuilding with regard to thermal insulation, fire insulation andsoundproofing.

However, it is also suited as a front sheet in adapting the lighting,interior design or acoustics of individual rooms to their specificpurpose. Finally, the large hollow spaces between the raw ceiling andthe FC are used to cover the laying/integration of pipelines, wiring andinlets and outlets of various building engineering installations.

3. FC Reguirements

High demands are made on false ceilings respectively on the usuallyplane components of which they are composed in three ways:

3.1 Structural:

(a) high stability, although, light weight,

(b) smooth, resistant surface,

(c) light, reversible mounting.

3.2 Structural acoustical:

(a) great mass in relation to the surface (5-10 kg/M²).

(b) closed, seamless modular design (50-200 cm)

(c) fibrous/porous hollow space dampening (50-100 mm)

3.3 Room acoustical:

(a) high degree of perforation (20-40%)

(b) fibrous/porous absorber surfacing (10-50 mm)

(c) great suspension height (20-50 cm).

Which of the partially contradicting demands is given precedence dependson the respective function of the room. However, some fundamentalproblems with FC systems remain unsolved if they are simultaneouslysupposed to be effective as acoustical ceilings:

4. Drawbacks of Conventional FCs

Even if the FC is supposed to only cover the installations accommodatedin the hollow space of the ceiling and itself soundproof the room asdescribed in Frick et al. or in "Trockenbau" July 1992 "Heiss-umkampfteKuhle", the mineral fiber panels and mats widely utilized as sheetcomponents, ceiling surfacing and hollow space dampening seem to bedisadvantageous and obstructive due to their

mechanical sensitivity during mounting and installation,

health hazard in rooms requiring high health standards,

physiological effects due to abrasion and shedding of fibers.

FIG. 1 shows a conventional reactive absorber according to Frick et al.,with a) representing a panel resonator, b) a Helmholtz resonator andchart c) the degree of absorption.

The conventional drop and view protection by means of foils havinglittle mass and panels having holes with a high degree of perforation(for room acoustical reasons) contradict the structural requirements tohave a not too light front sheet that is as closed as possible on theside facing the room.

The great suspension height of acoustical ceilings required for roomacoustical reasons for the absorption of low frequencies according toFrick et al. often contradicts the structural acoustical requirement ofsmall transverse transmission via the hollow space of the ceiling to theadjacent rooms even if the hollow space is filled like a kind ofsoundproofing with a large amount of fibrous or porous dampeningmaterial.

However, if the FC is to serve not only decorative and acousticalpurposes, but also to simultaneously assume other building engineeringfunctions as a (low pressure) ventilation ceiling, (radiation) heatingceiling or (surface) cooling ceiling, the fibrous/porous dampeningmaterial hitherto essential from an acoustical point of view has a majordrawback: it would not only obstruct mounting and installation but alsoobstruct maintenance and operation of the installations. Therefore,there is an urgent need for FC systems that meets the room andstructural acoustical needs without any use of porous absorbers and atthe same time accommodates the structural requirements better thanconventional acoustical ceilings.

5. Alternative ceiling panel sound absorbers

Conventional acoustical ceilings almost exclusively utilize passive(porous/fibrous) absorbers (Trockenbau July 1992). In order for theairborne soundwaves to be able to penetrate the dampening materialunhindered, the ceiling panels have to have a high degree of perforation(15-50%). They can only guarantee a respectively low airbornesoundproofing to the ceiling hollow space. Conventional reactive(panel/foil/Helmholtz) absorbers according to FIG. 1 require closedhollow spaces which again have to be filled with dampening material inorder to achieve even moderate wideband absorption. Although so-calledmembrane absorbers according to the requirements of FIG. 2 (cupstructure) and FIG. 3 (membrane absorbers) and as described in Fuchs, H.V. "Zur Absorption tiefer Frequenzen in Tonstudios. RundfunktechnischeMitteilungen rtm 36 (1992), H., 1, p 1-11" obviate the use ofporous/fibrous material, they on the other hand still need 5-10 cm deephollow chambers. Due to their three-shell construction on a relativelysmall mesh (10-20 cm) honeycomb structure, they are also much toocomplicated and expensive as a FC component for normal acousticalceilings. However, the latter can at the most be used as fully enclosedmetal cassettes in the hollow space of the ceiling or an integrated FCcomponent to supplement the absorption for low frequencies in rooms withspecial room acoustical needs.

SUMMARY OF THE INVENTION

The object of the present invention is to create a fiberfree acousticalfalse ceiling which absorbs wideband frequencies.

The FC component on the basis of staggered plane panels as resonancedampers presented herein combines the properties of microperforated andmembrane absorbers in that

although it has a practically smooth, closed surface facing the room,

the side facing the hollow space does not need own hollow chamber orhoneycomb structures,

completely obviates the use of porous/fibrous materials.

The new ceiling absorber panels can be utilized suspended as a ceilingfront immediately before respectively as a FC from the massive ceilingin all the fields of application detailed under 1. as well as can beprovided with all the properties and functions specified under 1. and 2.without possessing the drawbacks mentioned under 4.

The acoustical advantages of the FC system are set forth in thefollowing:

(a) False ceiling as front sheet

Fiberfree FC as a front ceiling (FIG. 10) for increasing airborne andfootfall soundproofing of the massive ceiling

made of thin panels 1, 6 of great density having sufficient surface mass(5-10 kg/m² ; e.g., metal, plastic, wood) in which soundwaves cannotexcite vibrations,

having evenly or unevenly disposed small (<2 mm) holes and lowhole-surface portion (<2%),

braced on the hollow space side by bands, ribs 2 (FIG. 10b),

in such a manner that the passage of sound through the holes remainsneglected and sagging of the ceiling panels is prevented even if thereare large grid fields (upto 200 cm) respectively between the respectivesuspenders.

(b) False ceiling as sound absorbers for the room-side sound field

Fiberfree FC as an acoustical ceiling (FIG. 10) for noise reduction andcontrolling room acoustics

made of thin panels 1, with the air in the holes in the panels togetherwith the air in the ceiling hollow space 11 executing dampened naturalvibrations, preferably at medium and high frequencies, excited by theroom-side sound field,

with panels 1, having evenly or unevenly disposed holes (<2 mm; andhole-surface portion <2%), in which the air together with the air in thehollow space respectively in the hollow space formed by the bracing 2executes dampened vibrations, preferably in the medium and highfrequencies, in the holes excited by the room-side sound field,

(c) False ceiling as soundproofing for the airborne sound-transverseconduction in the ceiling hollow space

Fiberfree FC as soundabsorbing framing of the ceiling hollow space as asound transmitting channel which executes dampened vibrations in a widefrequency range excited by the channel-side sound field and therebycontributes to reducing transverse transmission to the adjacent roomlike the dampening mechanisms described in (b).

The FC component made of even, room-side microperforated, high-densityceiling panels permits complete industrial manufacturing. The extremelysmall holes permit complete vision protection, the visual impression ofa closed ceiling surface and possibilities of decoratively loosening itup.

Preforms of any desired design as reflectors for illumination, inletsand outlets for ventilation and radiators can be made from the fiberfreepanel components without having to relinguish their acousticaleffectivity.

Microperforated FC systems c an me et the highest sanitary requirements,because

no porous/fibrous dampening material is involved,

offers few opportunities for deposition,

can be wiped and disinfected on the outside and on the inside.

They possess practically ideal prerequisites for mounting, removal andremounting and are completely and inexpensively reversible due to theirsimple, homogenous installation. If the FC components are made of metalthey also comply with the present trend in cooling adminstrationbuildings and assembly halls in summer: with so-called "coolingceilings" made of largely standardized metal components high ventilationpower consumption, which make up to 50% of the operational costs ofconventional air-conditioning, can be easily saved. Therefore itcontributes to lowering CO₂ emissions and eliminates an often verytroublesome source of draft, noise polution and allergies in homes andat the workplace. In the thermal insulation (e.g., aluminium-coveredhigh resistance foam) disposed over the coolant (i.e., water) pipesystem the spacing between the cooling lamina and the insulation, thethickness of the lamina, the diameter of the holes, and the number ofholes per m² can be tuned to each other in such a manner that optimumadaption to the reverberation period of the room or to the emissionspectrum of the sound sources set up in it can be achieved. Thefiberfree, microperforated FC ceiling also offers distinct advantageswith regard to heating and ventilation ceilings compared to theconventional systems.

FC components can be installed in a one-sheet, two-sheet or multi-sheetmanner. As simple front sheets, they may be completely even and smoothas well as be provided with a decorative pattern and reinforcingbeading, edging and folding. If the FC is designed as a suspendedcoffered ceiling, the hollow spaces of the coffers can be constructed asventilation channels. The actual rear wall of the coffered ceiling canbe advantageously designed from an acoustical and functional vantagepoint in such a manner that

varying adjacent hollow space depths are created for widening theabsorption effect,

recesses and molds can be created on the rear side of the ceiling in theactual hollow spaces of the ceiling for holding interior wiring orinstallation components,

fresh air, exhaust air and distribution channels can be created on thetop side of the ceiling in the coffer hollow space by means of molds andpartitions.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention as it is illustrated in FIGS. 8,9, 10, 11 is compared to the state of the art according to FIGS. 1 to 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts, as already briefly explained in the preceding, areactive absorber.

FIG. 1a shows a panel resonator in which the panel vibrates as a massbefore the air cushion like a spring, however requiring porous material,e.g. as edge damper in order to obtain a somewhat wideband dampeningbehavior such as in 1c.

In so-called foil absorbers according to DE 27 58 041 as shown in FIG.2, in a very complex cup structure, it was possible to excite a greatnumber of varying panel vibrations in different frequencies in such amanner that an all told wideband absorption spectrum is obtained atmedium frequencies even without the use of porous materials.

With the so-called membrane absorber, e.g. according to DE 35 04 208 andDE 34 12 432, it was for the first time possible to set up panel andHelmholtz resonators in succession in such a manner that multiplevibrations coupled via multiple air layers and holes already becomerelatively wideband excitable in a completely plane component. If arelatively thin plane layer (1-5 mm) of porous material is attachedbefore the ceiling membrane of this reactive absorber, as shown in FIG.3, an increase in absorption at high frequencies can be achievedaccording to FIGS. 4 and 5.

In FIG. 3, 15 stands for the ceiling membrane, 16 for the porousmaterial with a watertight cover 17 respectively with a mechanicalprotective cover 18. Below the ceiling membrane 15 is the perforatedmembrane 14 and at a distance the rear wall 12. The ceiling membrane,the perforated membrane and the rear wall are components that canvibrate, thus not rigid panels. The membranes are excited to vibrate andthey thereby draw the energy from the sound. The holes in the perforatedmembrane 14 vary between 3-10 mm. 13 stands for the walls of thehoneycomb structure, 11 for the hollow space, which usually is filledwith air. This membrane absorber may also be fabricated as a module. Themembranes 12, 14, 15 and 13 may be made of plastic or metal.

Furthermore, it is state of the art to cover large-volume porousabsorbers with perforated panels, with however the perforated panelsonly intended as mechanical protection. These porous absorbers are,e.g., pressed mineral fiber panels which are placed behind the suspendedfalse ceiling, with for practical reasons an aluminium foil being gluedonto these fiber panels or they being wrapped in a plastic foil. As itis known that penetration of soundwaves into the passive absorber islargely prevented by the foil, it is made "sound permeable" with amultiplicity of small holes by means of "perforation".

FIG. 6 shows the absorption spectrum according to Maa, D. Y. "Theory andDesign of Microperforated Panel Sound Absorbing Constructions", ScientiaSinica 18 (1975), H. 1, 55-71, with a microperforated panel beingdisposed before a rigid wall. Hitherto, however, this theoreticalresearch has not found technical application anywhere.

Up to now, only in the case of the aforementioned membrane absorbersaccording to FIG. 3 has it been possible to excite very specific naturalvibrations of the plane membranes which adapt well to the honeycombstructure disposed behind it and thereby being able to utilize it forthe desired absorption. In the case of the panel resonators with theirthick and therefore rigid panels hitherto employed in acoustics, thefrequencies of the "higher modes" of the panels before the respectiveair cushion are far above the frequency of the "basic mode" so that theyhave hitherto never been utilized for absorption of sound energy fromthe room. If these membrane absorbers are manufactured for flowchannels, e.g., in air conditioners, the panels are usually manufacturedthinner. The soundwaves in the channel are "swallowed" from the startmuch stronger far above the mass/spring resonance frequency by thealternately (about the channel) disposed purely passive absorbers thanby any higher modes of the panels themselves. Even if the latter couldbe excited in an interesting frequency range near the basic frequencycorresponding to the panel dimensions, these vibrations would not beable to develop properly at all due to the mineral-wool filling pressingagainst the full surface on one side. This was probably also the reasonwhy it has not been attempted to make higher modes in themicroperforated absorber according to FIG. 6 excitable with the aim ofwidening the effective frequency range.

Compared to this state of the art, the present invention relates to afalse ceiling having at least one microperforated metal panel or amicroperforated plastic panel before a non-vibrating wall 5 or rear wall7 which does not need the disposal of any sound swallowing elements oradditional porous or fibrous dampening materials in the air space.

Countless false ceilings having perforated metal panels are described in"Trockenbau" July 1992, in which "a sound swallowing backing made ofmineral wool for adaption to the acoustical requirements" (p. 2, lines24-26), which (the mineral wool) lies immediately with its whole surfaceon the panels having holes. The applicant of the present invention hasrepeatedly measured such systems in an acoustic room, because they areemployed in industry as false ceilings. FIG. 7 shows such a system withits absorbtion spectrum, the system having 0.5 mm thick steel sheets,2.5 mm hole diameter and 16% hole-surface portion, with the sheet beingdisposed about 200 mm below the ceiling. One can see that the nonwovenmaterial has a considerable proportion of the absorption in the higherfrequency ranges. The absorption frequency f../4=Co/4D (with Co=soundvelocity and D the space between the panel and the rear wall) has asexpected an increased absorption compared to the frequency ../2. Thisindicates that the achieved absorption is due to the dampening materiallying on the false ceiling. The air in the holes of the false ceilingtransmits only the sound vibrations of the soundwaves incident on themetal sheets having holes into the dampening material lying behind it.It is not until there that the sound energy is converted into heat bythe friction on the fibers or in the pores of the dampening material andthe sound energy is reduced thereby.

The problems involved with conventional sound absorbers, in particular,in view of the fact that recent research results indicate that the sounddampening material, e.g., rock wool or glass wool, is carcinogenic aswell as moisture absorbent, dust forming and abrasive, have led to asearch of new possible ways of sound dampening. On the other hand, themembrane absorbers have been known for quite some time. However, as theyare more expensive than the relatively more economical materials made ofrock wool or glass wool, they could not prevail. Moreover, membraneabsorbers, whether in their cup-shaped manner of design or in theprevious manner of construction with cleaved surfaces, in order to widenthe absorption spectrum, are relatively complicated and thereforeexpensive.

In comparison, the invented false ceiling is simple to manufacture,simple to mount and inexpensive, because it is only composed of finelyperforated metal sheets and the laterally bordering surfaces of the airspace and the plane rear wall respectively panel. The holes having adiameter of 0.2-3 mm, preferably less than 2 mm, more preferably 0.2-0.8mm, most preferably 0.4-0.8 mm are not intended as "openings" for asunimpeded as possible entry of sound energy into the air space betweenthe false ceiling and ceiling. The, for the invented purpose, extremelysmall hole-surface portion of maximal 5%, preferably less than 4%, morepreferably 0.5-3%, most preferably less than 2%, would be even lesssuited for the (passive) transmission of sound energy from the room intothe intermediate space than the openings according to the state of theart, because these have a hole-surface portion between 15-50%. Insteadthe air in the holes of the microperforated metal sheet according to theinvention in conjunction with the air cushion in the intermediate spaceacts like a very special mass-spring vibration system, which can be madeto excite vibrations in the respectively interesting frequency range bythe sound field (reactive) incident on the microperforated metal sheet.The tuning to the respective frequency range occurs by the completelypurposeful selection of geometric parameters, in particular thethickness of the perforated metal sheet, thickness of the air space, thediameter of the holes, the spacing of the holes, the shape of the holes,the proportion of the perforation in the overall surface of theperforated metal sheet and the shape of the metal sheets.

In particular, the selection of the hole configuration not onlydetermines the frequency range of the absorption but also theeffectivity of the absorbers in this frequency range. The necessarydampening is not achieved according to FIG. 1a or FIG. 7 by attachingadditional porous or fibrous "swallowing materials", but ratherexclusively by friction of the air particles on the walls of the smallholes. The desired frequency range and the required friction cantherefore be optimumly adapted to the respective application in such amanner that almost total absorption of the incident sound energy becomespossible. The panels are constructed so thick and stable that incidentsoundwaves cannot excite vibrations in them. Without themicroperforations of the invented type, the panels, to the extent thatthey are designed able to vibrate as shown in FIG. 8, would resonatelike a spring-mass system at most at very low frequencies and onlynarrowband according to the interrupted curve 1 and absorb thereby thesound. On the other hand, the microperforation, curve 2, results in arelatively wideband absorption at medium and high frequencies accordingto FIG. 8, because the light air in the holes resonates as mass with theair in the hollow space as the spring. With two successively disposed,rigid microperforated panels, as FIG. 9 shows, permits achieving an evenwider absorption spectrum without having to add additional dampeningmaterial or stationary components like a resonator having to resonate.

FIGS. 10a-f show the invented false ceiling, with FIG. 10e showing thefalse ceiling as a module which can then be attached as a false ceilingin a coffered manner under the ceiling.

In FIG. 10, 1 and 6 stand for the plane microperforated panel made ofsheet metal or hard plastic having holes 4, and 7 stands for avibratable panel as the rear wall of the module. 3b stands for the rigidframe of the module, and 11 stands for the hollow spaces or intermediatespaces filled with air. 3 are the suspensions and 3a, e.g., beams or asubconstruction for supporting the false ceiling respectively frontsheet. As the panels or modules were delivered in units of approximately1 square meter, varying spacings D of the false ceiling to the rear wallcan be realized via the suspensions 3 or subconstruction 3a, whereby theabsorption spectrum is widened. 2 stand for the reinforcements of thepanels 1, 6, which of course can also be disposed over the entire lengthand width of the panels in such a manner that it does not vibrate.

FIG. 11 shows the spectrum of microperforated panels made of aluminiumwith a thickness of the panel t of 0.15 mm, hole diameter of 0.16 mm,hole spacing of 1.2 mm and thickness of the air layer in theintermediate space between the panel and the rear wall or the ceiling of600 mm and a hole-surface portion p of 1.4% given by the diameter of theholes and the spacing.

With a desired resonance frequency of f_(R) =54×10³ √α/D.f.K_(m)according to Maa's theory, with σ the hole surface/overall surface, Dthe air layer thickness in the intermediate space and K_(m) a constant,which is proportional to the hole diameter multiplied by the root of f,the parameters panel thickness, hole surface portion respectively thenumber of holes with a specific hole diameter and air space D can bevaried within certain limits. Thus with an aluminium panel 3 mm thick, ahole-surface portion of p=1.4 and an air space of D=50 mm results in ahole diameter of 0.45 mm. If the holes are of a uniform size, but thenumber of holes is increased, according to the theory the resonancefrequency shifts to higher frequencies. This can also be achieved withsmaller holes. Furthermore, a widening of the spectrum is achieved ifthe panel is slightly curved downward as shown in FIG. 10F, e.g., with apanel width of 1000 mm and a curvature c of 60-80 mm.

What is claimed is:
 1. A false ceiling for rooms in buildings, which isdesigned to absorb soundwaves, comprising a perforated panel havingsufficient construction so that sound waves in the building do notexcite vibrations in the panel and having a multiplicity of holes havinga diameter d of 0.2-3 mm and a hole to surface portion of less than 4%,and suspensions or subconstructions for attaching the perforated panelto the buildings, wherein air in said holes forming with air in hollowspaces situated thereabove a spring-mass system and wherein additionalporous or fibrous damping material is not included.
 2. A false ceilingaccording to claim 1, wherein said holes have a hole to surface portionof less than 2%.
 3. A false ceiling according to claim 1, whereinmultiple panels are provided and said panels are disposed at anincreasing distance D in relation to the ceiling.
 4. A false ceilingaccording to claim 1, wherein said panels are composed of plastic,composites or metal.
 5. A false ceiling according to claim 1, furthercomprising reinforcements in order to prevent sagging of said panels. 6.A false ceiling according to claim 1, wherein said panels are attachedto a lateral frame and a plane rear wall designed as a module.
 7. Afalse ceiling according to claim 1, wherein said hole to surface portionis 0.5 to 3%.
 8. A false ceiling according to claim 2, wherein saidholes have a diameter of 0.2-0.8 mm.
 9. A false ceiling according toclaim 2, wherein said holes have a diameter of 0.4-0.8 mm.
 10. A falseceiling according to claim 7, wherein said holes have a diameter of0.4-0.8 mm.
 11. A false ceiling according to claim 1, wherein saidperforated panel has a downward curvature.